Ureteral stents with waveform interlayers and interstitching

ABSTRACT

Ureteral stents include a tubular body defining a lumen and have a distal kidney section, a proximal bladder section, and a ureter section between the distal and proximal sections. The tubular body has a first material layer and a stiffening member that extends through at least a portion of a length of the tubular body. The stiffening member has a stiffness characteristic that varies along its length. This enables different portions of the stent to have different stiffness characteristics that preferably are optimized for the location of those portions within the human body. The stiffening member may be at least a second material layer having a second stiffness that is different from a first stiffness of the first material layer, and/or at least one strand of material that is embedded within the first material layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 14/077,807, filed Nov. 12, 2013, the contents ofwhich are incorporated herein by reference.

BACKGROUND

The invention relates to ureteral stents.

A ureter is a tubular passageway in the body that conveys urine from akidney to a bladder. Ureteral stents are used to facilitate urinarydrainage from the kidney to the bladder in patients having a ureteralobstruction or injury, or to protect the integrity of the ureter in avariety of surgical manipulations. Ureteral stents are typically about30 cm long, hollow catheter-like devices made from a polymer and placedwithin the ureter with the distal end residing in the kidney and theproximal end residing in the bladder. Ureteral stents function bychanneling the flow of urine from the kidney to the bladder. One or bothends of a ureteral stent may be coiled in a pigtail shape to prevent theupward and/or downward migration of the stent due to patient movement.For example, the ureter may stretch up to 5 cm in either directionduring a patient's normal bodily movements, such as movement duringbreathing. If the stent is not sufficiently anchored, this may result instent migration and displacement.

Another factor to be considered relates to tissue irritation caused bythe stent. A stent may cause tissue irritation due to the relativemovement between the stent and the ureter during natural stretching ofthe ureter, even when the stent is properly anchored. A typicalsemi-rigid, anchored stent is unable to adjust for the natural extensionand contraction of the ureter during bodily movements, resulting inpressure and irritation of the ureter and surrounding tissue.

Regions of tissue most vulnerable to stent-induced irritation includethe kidney, the renal pelvis, the sensitive bladder tissue in thetrigonal region, and tissue of the ureteral vesicle junction leadinginto the bladder. Irritation may be caused by the static or dynamiccontact of the semi-rigid stent with sensitive tissues of the body, suchas the kidney and the renal pelvis. Chronic trigonal tissue irritationmay result from contact of tissue by the bladder-anchoring features ofthe stent, for example, pigtails at the stent ends. Irritation problemsare of concern regardless of the duration of use of the stent.Irritation is of particular concern, however, when use of a stent isrequired over a long time period.

Another problem associated with ureteral stents is urine reflux and painduring urine voiding. On the initiation of voiding, the bladder wallmuscles contract causing the pressure inside the bladder to increase.Because a typical ureteral stent holds the ureteral orifice open,increased bladder pressure during voiding is transmitted to the kidneythrough the stent, causing urine reflux and flank pain.

SUMMARY

Many factors thus should be considered when designing a ureteral stent.Such factors include the function to be performed by different parts ofthe stent, such as anchoring, maintenance of an open-flow condition,etc., and comfort. In particular, it is desirable to make a ureteralstent that is easy to insert, comfortable at all times, exhibits goodcoil recovery (the tendency of the stent ends to return to theoriginally-designed coiled state after having been straightened, forexample, during insertion), remains anchored during normal bodilymovements, provides for suitable flow of urine, is easily removable andavoids fracture during insertion, use and removal. The invention relatesto various designs for a ureteral stent that facilitate many or all ofthe above goals by exhibiting varying characteristics along the lengthof the stent.

Ureteral stents according to embodiments of the invention include atubular body defining a lumen and having (i) a distal kidney section tobe placed in or near a patient's kidney, (ii) a proximal bladder sectionto be placed within the patient's bladder, and (iii) a ureter sectionbetween the distal and proximal sections to be placed within thepatient's ureter.

According to a first aspect of the invention, the tubular body has afirst material layer and a stiffening member that extends through atleast a portion of a length of the tubular body. The stiffening memberhas a stiffness characteristic that varies along its length. Thisenables different portions of the stent to have different stiffnesscharacteristics that preferably are optimized for the location of thoseportions within the human body.

According to some embodiments, the stiffening member has a firststiffness characteristic in the distal kidney section, and a secondstiffness characteristic that is different from the first stiffnesscharacteristic in one or both of the ureter section and the proximalbladder section. According to other embodiments, the stiffening memberhas a first stiffness characteristic in the proximal bladder section,and a second stiffness characteristic that is different from the firststiffness characteristic in one or both of the ureter section and thedistal kidney section. According to other embodiments, the stiffeningmember has a first stiffness characteristic in the ureter section, and asecond stiffness characteristic that is different from the firststiffness characteristic in one or both of the distal kidney section andthe proximal bladder section. In some embodiments, the stiffening memberhas different stiffness characteristics in each of the distal kidneysection, the ureter section, and the proximal bladder section.

According to some embodiments, the stiffening member includes at least asecond material layer having a second stiffness that is different from afirst stiffness of the first material layer. A shape of an interfacebetween the first material layer and the second material layer may varyalong the length of the tubular body in order to vary the stiffnesscharacteristic along the length of the tubular body. A thickness (orcross-sectional area) of the first material layer and of the secondmaterial layer may vary along the length of the tubular body in order tovary the stiffness characteristic of the tubular body. Furthermore, astiffness of the second material layer may vary along the length of thetubular body in order to vary the stiffness characteristic of thetubular body.

According to some embodiments, the stiffening member includes aplurality of additional material layers having different stiffnesscharacteristics, with a number of the additional material layers varyingalong the length of the tubular body. This also results in the tubularbody having different stiffness characteristics along different portionsof its length.

According to some embodiments, the stiffening member includes at leastone strand of material that is embedded within the first material layer.A thickness of the at least one strand of material may vary along thelength of the tubular body so as to vary the stiffness characteristic ofthe tubular body along its length. According to some embodiments, across-sectional shape of the at least one strand of material variesalong the length of the tubular body in order to vary the stiffness ofthe tubular body. According to other embodiments, a path of the at leastone strand of material varies along the length of the tubular body so asto vary the stiffness characteristic of the tubular body along itslength. According to some embodiments, a number of the strands of thematerial varies along the length of the tubular body in order to varythe stiffness characteristics of the tubular body along its length.

According to some embodiments, first and second strands having stiffnesscharacteristics that differ from each other are embedded within thefirst material layer. The first strand can be located on a first side ofthe tubular body relative to a longitudinal axis of the tubular body.The second strand can be located on a second side of the tubular bodyrelative to the longitudinal axis of the tubular body. Providing thefirst and second strands with different stiffness characteristics onfirst and second sides of the tubular body may cause the tubular body tohave a preferential bending direction. Such a structure is particularlyuseful for forming the coiled ends of the stent.

According to another aspect of the invention, the tubular body includesat least a first material layer and a second material layer each ofwhich extend through at least a portion of a length of the tubular body.The first material of the first material layer is different from thesecond material of the second material layer. In addition, at least oneof a thickness, a cross-sectional shape and a longitudinal-sectionalshape of one or both of the first and second material layers variesalong the length of the tubular body. For example, the at least one ofthe thickness, cross-sectional shape and longitudinal-sectional shape ofone or both of the first and second material layers can vary between thedistal kidney section, the proximal bladder section, and the uretersection of the tubular body.

According to another aspect of the invention, the tubular body includesa first material layer and at least one strand of material embeddedwithin the first material layer and that extends through at least aportion of a length of the tubular body. In addition, at least one ofthe following varies along the length of the tubular body: (a) athickness of the at least one strand of material, (b) a cross-sectionalshape of the at least one strand of material, (c) a path of the at leastone strand of material, and (d) a number of the strands of the material.For example, the at least one of (a) the thickness of the at least onestrand of material, (b) the cross-sectional shape of the at least onestrand of material, (c) the path of the at least one strand of material,and (d) the number of the strands of the material, can vary between thedistal kidney section, the proximal bladder section, and the uretersection of the tubular body.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary embodiments of ureteral stents according to aspects ofthe invention will be described in detail with reference to thefollowing drawings in which:

FIG. 1 shows a ureteral stent according to embodiments of the invention;

FIG. 2 shows a ureteral stent disposed within a patient's kidney, ureterand bladder;

FIGS. 3A-3H are cross-sectional views of stents showing differentmaterial layers and different shapes of the interface between thosematerial layers;

FIGS. 4A-4F are cross-sectional views of multi-material-layer stentstaken through a plane that is parallel to the longitudinal axis of thestent;

FIGS. 5A-F are cross-sectional views of stents showing different numbersof strands used as a stiffening member, the cross-section taken througha plane that is orthogonal to the longitudinal axis of the stent;

FIGS. 6A-6L show cross-sectional shapes of strands that may be used; and

FIGS. 7A-7C are cross-sectional views, taken through a plane that isparallel to the longitudinal axis of the stent, of stents having one ormore strands of material therein.

DETAILED DESCRIPTION OF EMBODIMENTS

The invention relates to ureteral stents configured for improved patientcomfort. In particular, the stents have different stiffnesscharacteristics along the length of the stent. As a typical stent has adistal kidney section, a proximal bladder section and a ureter sectionbetween the distal and proximal sections, one or more of those sectionsare configured to have a stiffness characteristic that differs from thestiffness characteristic of one or both of the other sections. Inaddition, the stiffness characteristic can vary along all or part of asection, and also can vary in the region where two different sectionsjoin each other. In order to achieve the difference in stiffnesscharacteristics, one or more stiffening members is/are included along atleast a part of the length of the stent, and the stiffening member isconfigured to have a stiffness characteristic that varies along itslength. According to some embodiments, the stiffening member is amaterial layer that differs in stiffness characteristic compared to afirst material layer that forms a majority of the stent. Furthermore,the stiffness characteristic of the stent can be varied by varying atleast one of a thickness, a cross-sectional shape and alongitudinal-sectional shape of one or both of the first and secondmaterial layers along all or part of the length of the stent. Accordingto some embodiments, the stiffening member is one or more strands ofmaterial embedded within the first material layer that forms themajority of the stent. In order to vary the stiffness of the stent alongits length, at least one of the following varies along the length of thetubular body: (a) a thickness of the one or more strands of material,(b) a cross-sectional shape of the one or more strands of material, (c)a path of the one or more strands of material, and (d) a number of thestrands of material.

FIG. 1 shows a ureteral stent according to one embodiment of theinvention. The stent 100 is a tubular, catheter-like body 105 having aninternal lumen 106 (not shown in FIG. 1) extending from the distal end142 to the proximal end 122. There can be a distal-most opening at thedistal end 142, or the distal end 142 could be a rounded, closed end.Similarly, the proximal end 122 could have a proximal-most opening orthe proximal end 122 could be rounded and closed. The tubular body 105includes openings 110 along its length through which liquid such asurine may flow through the stent 100. The stent 100 essentially hasthree sections: (1) distal kidney section 140 that will be locatedwithin the kidney during use, (2) proximal bladder section 120 that willbe located within the bladder during use, and (3) ureter section 180that will be located within the ureter during use and which is disposedbetween the distal kidney section 140 and the proximal bladder section120. In addition, strings 190 may be provided at the proximal end 122 ofthe proximal bladder section 120 for use in removing the stent 100 fromthe patient.

FIG. 2 shows one manner in which the stent 100 can be disposed withinthe human body. Distal kidney section 140 will be disposed within thepatient's kidney 200, proximal bladder section 120 will be disposedwithin the patient's bladder 300, and ureter section 180 will bedisposed within the patient's ureter 400. As noted above, in order toincrease patient comfort, it is desirable to configure the differentsections of the stent to have different stiffness characteristics. Forexample, it can be desirable to have the distal kidney section 140 bethe stiffest section of the stent, the proximal bladder section 120 bethe least stiff (softest) section of the stent, and the ureter section180 be a transition zone from stiffest (near the distal kidney section140) to the least stiff (near the proximal bladder section 120). Thedistal kidney section 140 can be designed to resist stent migration bymaking it stiffer and providing a higher coil strength than otherportions of the stent. The proximal bladder section 120 can be designedto focus on comfort, for example, by making it softer than otherportions of the stent. The ureter section 180 can be designed to improvecomfort during insertion, however, this region is not as critical as thedistal kidney section 140 and the proximal bladder section 120.Different waveform interlayers (see FIGS. 3A-4F) and/or interstitchingstructure (see FIGS. 5A-7C) could be designed along the length of stent100 so that the stent 100 has a variable stiffness along its length tomeet a favorable stress concentration profile with comfort stentfeatures, specifically targeted at the different regions.

FIGS. 3A-3H relate to embodiments in which one or more stiffeningmembers in the form of a material layer is/are incorporated into thestent. Each of FIGS. 3A-3H is a cross-sectional view taken through thestent in a direction orthogonal to the longitudinal axis of the stent.Thus, in each of FIGS. 3A-3H, the internal lumen 106 is present in thecross-sectional view. The shape of the interface between two layers cantake many forms. For example, the interface shape can be polygonal (atriangle, a quadrilateral, a pentagon, a hexagon, etc.), conic (acircle, an ellipse, a parabola, a hyperbola, etc.), or the interface canhave different symmetries including but not limited to reflection,rotational, translational, roto-reflection, helical, non-isometric,scale and fractals.

FIG. 3A shows an embodiment in which three material layers 132, 134 and136 are present in at least part of the stent. The first material layer132 defines the outer surface of the stent 100. The first material layercan be, for example, various polyurethanes, polyolefins, silicone,and/or proprietary polymers (such as Tecoflex™, Tecophillic™,Carbothan™, Tecothane™, Pellethan™, C-Flex™, Percuflex™, Silitek™,etc.). The second material layer 134 is made from a material having astiffness characteristic different from the stiffness characteristic ofthe first material layer 132. The second material layer 134 can be, forexample, various polyurethanes, polyolefins, silicone, and/orproprietary polymers (such as Tecoflex™, Tecophillic™, Carbothan™,Tecothane™, Pellethan™, C-Flex™, Percuflex™, Silitek™, etc.). As can beseen from FIG. 3A, the interface between the first material layer 132and the second material layer 134 has a triangular shape incross-section. A third material layer 136 is the innermost layer suchthat it forms a surface of the lumen 106. The third material layer 136is made from a material different from at least the second materiallayer 134 (and in some embodiments different from both the firstmaterial layer 132 and the second material layer 134). The thirdmaterial layer 136 also can be selected from, for example, variouspolyurethanes, polyolefins, silicone, and/or proprietary polymers (suchas Tecoflex™, Tecophillic™, Carbothan™, Tecothane™, Pellethan™, C-Flex™,Percuflex™, Silitek™, etc.). As can be seen from FIG. 3A, the interfacebetween the second material layer 134 and the third material layer 136is circular. By varying the shape of the interfaces, as well as thecross-sectional area of each material layer along a length of thetubular body 105 of the stent 100, the stiffness of the stent can varyalong the length of the stent.

FIG. 3B shows an embodiment in which only two material layers are used.The first material layer 132 forms the outer surface of the stent 100.The second material layer 134 forms the lumen 106 of the stent 100. Theshape of the interface between the first material layer 132 and thesecond material layer 134 is square. As with FIG. 3A, differentmaterials are used for each of layers 132 and 134, the differentmaterials having different stiffness characteristics. Layers 132 and 134can be selected from the same materials described above in connectionwith FIG. 3A.

FIG. 3C shows an embodiment in which only two material layers are used.The first material layer 132 forms the outer surface of the stent 100.The second material layer 134 forms the lumen 106 of the stent 100. Theshape of the interface between the first material layer 132 and thesecond material layer 134 is hexagonal. As with FIG. 3A, differentmaterials are used for each of layers 132 and 134, the differentmaterials having different stiffness characteristics. Layers 132 and 134can be selected from the same materials described above in connectionwith FIG. 3A.

FIG. 3D shows an embodiment in which three material layers are used. Thefirst material layer 132 forms the outer surface of the stent 100. Thesecond material layer 134 is located radial inward of the first materiallayer 132. A third material layer 136 forms the lumen 106 of the stent100. The shape of the interface between the first material layer 132 andthe second material layer 134 is round. The shape of the interfacebetween the second material layer 134 and the third material layer 136also is round. As with FIG. 3A, different materials are used for each oflayers 132, 134 and 136, the different materials having differentstiffness characteristics. Layers 132, 134 and 136 can be selected fromthe same materials described above in connection with FIG. 3A.

FIG. 3E shows an embodiment in which only two material layers are used.The first material layer 132 forms the outer surface of the stent 100.The second material layer 134 forms the lumen 106 of the stent 100. Theshape of the interface between the first material layer 132 and thesecond material layer 134 is parabolic. As with FIG. 3A, differentmaterials are used for each of layers 132 and 134, the differentmaterials having different stiffness characteristics. Layers 132 and 134can be selected from the same materials described above in connectionwith FIG. 3A.

FIG. 3F shows an embodiment in which only two material layers are used.The first material layer 132 forms the outer surface of the stent 100.The second material layer 134 forms the lumen 106 of the stent 100. Theshape of the interface between the first material layer 132 and thesecond material layer 134 is elliptical. As with FIG. 3A, differentmaterials are used for each of layers 132 and 134, the differentmaterials having different stiffness characteristics. Layers 132 and 134can be selected from the same materials described above in connectionwith FIG. 3A.

FIGS. 3G and 3H show embodiments in which only two material layers areused. The first material layer 132 forms the outer surface of the stent100. The second material layer 134 forms the lumen 106 of the stent 100.The shape of the interface between the first material layer 132 and thesecond material layer 134 is irregular but symmetrical with respect totwo perpendicular axes that each are perpendicular to the longitudinalaxis of the stent. As with FIG. 3A, different materials are used foreach of layers 132 and 134, the different materials having differentstiffness characteristics. Layers 132 and 134 can be selected from thesame materials described above in connection with FIG. 3A.

The different cross-sectional configurations shown in FIGS. 3A-3H canvary along the length of the stent. For example, referring to FIG. 3A,layer 136 can be stiffer than layers 132 and 134, and further, layer 134can be stiffer than layer 132. The three layers 132, 134 and 136 can bepresent in the distal kidney section 140 so that the distal kidneysection 140 is relatively stiff. Only layers 132 and 134 may be presentin the ureter section 180 so that the ureter section 180 is less stiffthan the distal kidney section 140. Furthermore, the proximal bladdersection 120 could be the same as the ureter section (having layers 132and 134) or the proximal bladder section 120 may have only the materiallayer 132 so as to be less stiff than both the distal kidney section 140and the ureter section 180. Furthermore, the thickness of a layer (forexample, layer 136) could gradually decrease as one moves proximallyfrom the distal end of the ureter section 180 (or even from within theproximal end of the distal kidney section 140) until the layer 136terminates either within the ureter section 180 or within the distal endof the proximal bladder section 140. Also, two or more of the differentcross-sectional structures shown in FIGS. 3A-3H could be present withina single stent 100 at different longitudinal positions along the lengthof the stent 100.

FIGS. 4A-4F are cross-sectional views of multi-material-layer stentstaken through a plane that is parallel to the longitudinal axis of thestent. As shown in FIGS. 4A-4F, the interface between two layers canvary like a wave when viewed in the longitudinal direction of the stent.The wave form between two layers in the longitudinal direction includesbut is not limited to sine waves (FIG. 4A), square waves (FIG. 4B),triangle waves (FIG. 4C), saw tooth waves (FIG. 4D), Fourier Transformwaves (FIG. 4E), longitudinally straight interfaces (FIG. 4F). Thedifferent waveforms shown in FIGS. 4A-4F could be used along part of thelength of the stent to provide a stiffness characteristic associatedwith that waveform to the part of the length of the stent at which thewaveform is provided. Different waveforms shown in FIGS. 4A-4F alsocould be present within a single stent 100 at different longitudinalpositions along the length of the stent 100. The longitudinal profilesof stent 100 may vary along the length of stent 100, and differentcross-sectional shapes may be utilized along the length of stent 100.Various waveform profiles may be combined to form a single stentcombination design.

A comfort stent profile can be created using different waveforms andinterstitching to better match the elastic modulus of the urinary tractsystem. Typical modulus of elasticity (E) for soft tissue is betweenabout 0.3 kPa and 3 kPA. A design concept for stent 100 may be bestmatched for comfort through a matching with the elastic modulus of thetissues along the insertion path as well as the inserted position.

It may be more desirable to have a cross-sectional profile that wouldresult in a configuration which was relatively harder to bend in certainsections along the length of stent 100. For example, the cross sectionalprofiles of FIG. 3A or 3G may result in a stent which was relativelyharder to bend. It may also be desirable to have a cross-sectionalprofile that would result in a configuration which was relativelysofter, or easier to bend, in certain sections along the length of thestent 100. For example, the cross sectional profiles of FIG. 3D or 3Fmay result in a stent which was relatively softer, or easier to bend. Itmay also be desirable to have a cross sectional profile that wassomewhere in between hard and soft. For example, the cross sectionalprofiles of FIGS. 3B and 3C may result in a stent which was somewhere inbetween hard and soft in terms of bending rigidity.

It is contemplated that one might desire to have a more rigid stent atthe insertion end, or in the distal kidney section 140. One might desireto have a less rigid stent at the proximal bladder section 120. Onemight desire to have a stiffness somewhere in between for the uretersection 180. In particular at the ureteropelvic junction (UPJ),approximately where the distal kidney section 140 and the ureter section180 meet, and at the ureterovesical junction (UVJ), approximately wherethe ureter section 180 and the proximal bladder section 120 meet, onemight desire a cross sectional profile that would result in the mostcomfort as this is an area of irritation in more traditional stents.

Any of the transverse cross-section configurations shown in FIGS. 3A-3Hcan be combined with any of the longitudinal cross-sectionconfigurations shown in FIGS. 4A-4F.

FIGS. 5A-5F relate to embodiments in which one or more stiffeningmembers in the form of a strand of material layer is/are incorporatedinto the stent. A thickness of the at least one strand of material mayvary along the length of the tubular body so as to vary the stiffnesscharacteristic of the tubular body along its length. According to someembodiments, a cross-sectional shape of the at least one strand ofmaterial varies along the length of the tubular body in order to varythe stiffness of the tubular body. According to other embodiments, apath of the at least one strand of material varies along the length ofthe tubular body so as to vary the stiffness characteristic of thetubular body along its length. According to some embodiments, a numberof the strands of the material varies along the length of the tubularbody in order to vary the stiffness characteristics of the tubular bodyalong its length. Each of FIGS. 5A-5F is a cross-sectional view takenthrough the stent in a direction orthogonal to the longitudinal axis ofthe stent. Thus, in each of FIGS. 5A-5F, the internal lumen 106 ispresent in the cross-sectional view. The number of strands presentvaries in each of FIGS. 5A-5F.

In FIG. 5A, two strands 150A and 150B are present. Strands 150A and 150Bare disposed at intervals of 180 degrees around the circumference of thestent, although other arrangements are possible and contemplated herein.The tubular body portion of the stent 100 can be made from a singlematerial layer, such as the material layer 132 described above, or itcan have multiple material layers in all or only parts of the length ofthe stent as described above in connection with FIGS. 3A-3H. Strands150A and 150B are made of, for example, materials similar to thematerials described above in connection with layers 132, 134 and 136.The strands 150A and 150B could be identical to each other or could havedifferent stiffnesses, for example, by being made from differentmaterials or by having different diameters or cross-sectional shapes. Bymaking the strands 150A and 150B on opposite sides of the longitudinalaxis have different stiffnesses, the portion of the stent having thestrands will preferentially bend in one direction.

It is well known that during stent insertion, soft stent materials suchas silicone especially when in combination with a complex tortuousinsertion path may have some degree of insertion difficulty due topotential buckling as well as potential high surface friction. Thewaveform interlayer and interstitching stent design concept of thepresent invention could be used for overcoming the issue of insertionfor soft stent materials through materials selection, stent stressdesign at either dynamic load condition or static load condition(compression and tension) by varying both the cross-sectional profileand longitudinal profile using different materials for the differentsections within the cross sectional or longitudinal profiles. By way ofexample, it may be desired for the material durometer to be harder, inthe range of 10 to 50 Shore A, at the distal kidney section 140.Further, it may be desired for the material durometer to be softer, inthe range of 50 to 90 Shore A, at the proximal bladder section 120. Inthe ureter section 180, the material durometer may be somewhere inbetween the material durometer of the distal kidney section 140 and theproximal bladder section 120.

It is contemplated that a stent designer would desire a ureteral stentthat provided for a combination of increased comfort as well asresistance to buckling during insertion. Euler's formula for buckling,F=π²El/(KL)², where F is the maximum or critical force, E is Young'sModulus, I is the area moment of inertia, K is the column effectivelength factor, and L is the unsupported column length, could be a goodguideline for critical load design. The critical force which would causebuckling can be modified with a changing modulus of elasticity, changingsurface friction coefficient, changing moment of inertia, changingunsupported column length, and changing column effective length factor.Also, the strands can be positioned in only a portion of the length ofthe stent so that the portion has a stiffness characteristic associatedwith the strands. For example, if the strands 150A and 150B are stifferthan the material 132 making up the remainder of the stent, the strandscan be disposed only in the distal kidney section 140 so that the distalkidney section 140 is stiffer than the ureter section 180 and theproximal bladder section 120 of the stent. In addition, the number ofstrands within a cross-section of the stent can diminish as one movesproximally along the length of the stent so that the stent becomes lessstiff as one moves proximally. For example, two strands can be presentin the distal kidney section 140, one strand can be present in all or atleast the distal part of the ureter section 180, and the proximalbladder section 120 can be provided with no strand.

Other strand arrangements are possible. Some alternative arrangementsare shown in connection with FIGS. 5B-5F.

In FIG. 5B, three strands 150A, 150B and 150C are present. The strands150A-150C are disposed at intervals of 120 degrees around thecircumference of the stent, although other arrangements are possible andcontemplated herein.

In FIG. 5C, four strands 150A, 150B, 150C and 150D are present. Thestrands 150A-150D are disposed at intervals of 90 degrees around thecircumference of the stent, although other arrangements are possible andcontemplated herein.

In FIG. 5D, five strands 150A, 150B, 150C, 150D and 150E are present.The strands 150A-150E are disposed at intervals of 72 degrees around thecircumference of the stent, although other arrangements are possible andcontemplated herein.

In FIG. 5E, six strands 150A, 150B, 150C, 150D, 150E and 150F arepresent. The strands 150A-150F are disposed at intervals of 60 degreesaround the circumference of the stent, although other arrangements arepossible and contemplated herein.

In FIG. 5F, eight strands 150A, 150B, 150C, 150D, 150E, 150F, 150G and150H are present. The strands 150A-150H are disposed at intervals of 45degrees around the circumference of the stent, although otherarrangements are possible and contemplated herein.

FIGS. 6A-6L show cross-sectional shapes of strands 150 that may be used.The strands 150 can have various shapes in cross-section taken through aplane that is orthogonal to the longitudinal axis of the stent (and ofthe strand). The cross-sectional shape can be polygonal (triangle,quadrilateral, pentagon, hexagon, etc.), conic (circle, ellipse,parabola, hyperbola, etc.), or the strand cross-section can havedifferent symmetries including but not limited to reflection,rotational, translational, roto-reflection, helical, non-isometric,scale and fractals. In FIG. 6A, the cross-sectional shape is a triangle.In FIG. 6B, the cross-sectional shape is a square. In FIG. 6C, thecross-sectional shape is a pentagon. In FIG. 6D, the cross-sectionalshape is a hexagon. In FIG. 6E, the cross-sectional shape is an octagon.In FIG. 6F, the cross-sectional shape is a circle. In FIG. 6G, thecross-sectional shape is an ellipse. In FIG. 6H, the cross-sectionalshape is a parabola. In FIGS. 61, 6J, 6K and 6L, the cross-sectionalshape is formed by combining polygons with conic sections.

FIGS. 7A-7C are cross-sectional views, taken through a plane that isparallel to the longitudinal axis of the stent, of stents having one ormore strands of material therein. As shown in FIGS. 7A-7C, the radialposition of the strand (distance of the strand from the central,longitudinal axis of the stent) can vary like a wave when viewed in thelongitudinal direction of the stent. The wave form of the strand(s) inthe longitudinal direction includes but is not limited to triangle waves(FIG. 7A), sine waves (FIG. 7B) and square waves (FIG. 7C), althoughother waveforms such as waveforms shown in FIGS. 4D-4F are contemplatedherein. The broken line waveform for strand 150B in FIGS. 7A-7C meansthat the strand 150B can be different in stiffness from strand 150A.Such a structure gives the portion of the stent having the strands 150Aand 150B a preferential bending direction as discussed earlier. Thedifferent waveforms shown in FIGS. 7A-7C could be used along part of thelength of the stent to provide a stiffness characteristic associatedwith that waveform to the part of the length of the stent at which thewaveform is provided. Different waveforms shown in FIGS. 7A-7C alsocould be present within a single stent 100 at different longitudinalpositions along the length of the stent 100. Adjusting the waveforminterlayers and interstitching design choices along the length of theureteral stent 100 may provide a designer with more comfort featureflexibility in one stent design.

Any of the transverse cross-section configurations shown in FIGS. 5A-5Fcan be combined with any of the transverse strand cross-sections shownin FIGS. 6A-6L, and any of those combinations can be combined with anyof the longitudinal cross-section configurations shown in FIGS. 7A-7C.

In addition, any of the embodiments using multiple material layers canbe combined with any of the embodiments using one or more strands.

By providing the various configurations described above in differentsections of the stent, each stent section can be configured to havedesired characteristics in terms of stiffness. This improves the comfortlevel that can be achieved with a ureteral stent.

Materials selection as well as stent structure will be critical forcomfort stent design at the distal kidney section 140 and at theproximal bladder section 120. Ureter section 180 may be less critical asfar as the exact waveform interlayer or interstitching selected. Choiceof waveform interlayers and interstitching design concepts providesoptions for the comfort stent designer.

Stents having the structures described above preferably are made byco-extrusion processes. Co-extrusion processes are continuous processesthat are more robust and easier to modify than batch-type processes.

The illustrated exemplary embodiments are intended to be illustrativeand not limiting. Various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A ureteral stent comprising: a tubular bodydefining a lumen and having (i) a distal kidney section to be placed inor near a patient's kidney, (ii) a proximal bladder section to be placedwithin the patient's bladder, and (iii) a ureter section between thedistal and proximal sections to be placed within the patient's ureter;the tubular body having a first material layer and a stiffening memberthat extends through at least a portion of a length of the tubular body,the stiffening member having a stiffness characteristic that variesalong its length, wherein the stiffening member includes at least onestrand of material that is embedded within the first material layer. 2.The ureteral stent according to claim 1, wherein a thickness of the atleast one strand of material varies along the length of the tubularbody.
 3. The ureteral stent according to claim 1, wherein across-sectional shape of the at least one strand of material variesalong the length of the tubular body.
 4. The ureteral stent according toclaim 1, wherein a path of the at least one strand of material variesalong the length of the tubular body.
 5. The ureteral stent according toclaim 1, wherein a number of the strands of the material varies alongthe length of the tubular body.
 6. The ureteral stent according to claim1, wherein the at least one strand includes a first strand and a secondstrand having stiffness characteristics that differ from each other. 7.A ureteral stent comprising: a tubular body defining a lumen and having(i) a distal kidney section to be placed in or near a patient's kidney,(ii) a proximal bladder section to be placed within the patient'sbladder, and (iii) a ureter section between the distal and proximalsections to be placed within the patient's ureter; the tubular bodyhaving a first material layer and at least one strand of materialembedded within the first material layer and that extends through atleast a portion of a length of the tubular body, wherein at least one ofthe following varies along the length of the tubular body: (a) athickness of the at least one strand of material, (b) a cross-sectionalshape of the at least one strand of material, (c) a path of the at leastone strand of material, and (d) a number of the strands of the material.8. The ureteral stent according to claim 7, wherein the at least one of(a) the thickness of the at least one strand of material, (b) thecross-sectional shape of the at least one strand of material, (c) thepath of the at least one strand of material, and (d) the number of thestrands of the material, varies between the distal kidney section, theproximal bladder section, and the ureter section of the tubular body. 9.The ureteral stent according to claim 7, wherein the first strand islocated on a first side of the tubular body relative to a longitudinalaxis of the tubular body and the second strand is located on a secondside of the tubular body relative to the longitudinal axis of thetubular body so that the tubular body has a preferential bendingdirection
 10. The ureteral stent of claim 1, further comprising a secondmaterial layer which extends through at least a portion of a length ofthe tubular body.
 11. The ureteral stent of claim 10, wherein the atleast one strand includes a first strand and a second strand havingstiffness characteristics that differ from each other.
 12. The ureteralstent of claim 10, wherein at least one of a thickness, across-sectional shape and a longitudinal-sectional shape of one or bothof the first and second material layers varies along the length of thetubular body.
 13. The ureteral stent of claim 1, material durometer isin the range of 50 to 90 Shore A at the proximal bladder section.